Fluororubbers possess rubber elasticity, an inherent characteristic in common with other general-purpose rubbers, and are superior to the general-purpose rubbers in properties such as resistance to heat, oil and chemicals. Such properties provide various applications, including leakproof rubber parts represented by O-rings, packings and gaskets, rubber vibration insulators, belts and rubber-coated fabrics, as well as impact-absorbing stoppers for printer head controllers and HDD (hard disk drive) head controllers, more particularly stoppers fitted in HDD to prevent a malfunction of a reading arm.
The conventional fluororubbers have surface tackiness and high friction coefficient, and therefore the processing thereof often involves tack eliminating treatment on the crosslinked rubber surface. This increases the treatment cost.
The use of conventional fluororubbers as stoppers of storage devices in hard disk drives (HDD) causes a malfunction by the tackiness of the stopper to the arm. Furthermore, their damping properties are greatly dependent on temperature and the impact resilience is increased at high temperatures such that the stoppers do not absorb the vibration of the arm. As used herein, the stoppers are parts that define the range of movement (swing) to which an arm having a reading head at the tip can move from a standby position and that absorb the impact to prevent a malfunction of the arm after operation or standby.
Meanwhile, recent rubber stoppers that are increasingly used include magnet holder-type stoppers that incorporate a magnet in the rubber to fix the arm by magnetic attraction, and crush stop-type stoppers arranged on both sides of the arm. Performances required for such stoppers are mainly the following three:
(1) The stoppers exhibit excellent impact-absorbing properties when the arm collides therewith.
(2) The rubber stopper holds an arm end portion (metal) in close contact by the magnetic force or the like during standby but does not adhere thereto.
(3) The stoppers are clean.
The stoppers of common fluororubbers are generally satisfactory in terms of (1) the impact-absorbing properties and (3) the cleanness, but they have high tackiness and fail to satisfy the required performance (2).
To satisfy the required performance (2), JP-B-H04-37094 proposes a method in which the rubber surface is impregnated with a solution of a crosslinking agent and a crosslinking accelerator for fluororubber to perform re-crosslinking and thereby the surface tackiness is eliminated. This method, however, uses a large amount of solvent and is undesirable in view of adverse effects on the environment. Moreover, the solvent limits the impregnation, so that products, for example stoppers, have variations in performance and some cause a malfunction of HDD.
The present inventors studied diligently in order to solve the aforesaid problems, and have arrived at an uncrosslinked fluororubber composition that comprises a polyol-crosslinkable fluororubber in combination with a specific crosslinking agent, crosslinking accelerator, calcium hydroxide and optionally magnesium oxide, wherein the crosslinking accelerator has a weight ratio (R) to the crosslinking agent (crosslinking accelerator/crosslinking agent) in the range of 0.9 to 5, and that when heat treated under specific conditions (the heat treatment may be preceded by polyol crosslinking and preforming according to need), the crosslinking accelerator in the fluororubber composition (or a preform) favorably migrates to the superficial layer to increase the crosslinking density of the rubber surface such that the rubber surface displays reduced frictional properties and tackiness and the fluororubber formed product exhibits low impact resilience.
Specifically, the present inventors have found that the greater the crosslinking accelerator (such as an organic quaternary phosphonium salt)/crosslinking agent ratio is within the range of 0.9 to 5, the larger the amount of the crosslinking accelerator migrating to the rubber surface, with the result that the crosslinking density increases in the surface of rubber formed product while that of the entire rubber formed product decreases, leading to lowered impact resilience.
The present inventors have also found that the fluororubber composition in which the crosslinking accelerator (such as an organic quaternary phosphonium salt)/crosslinking agent ratio is higher than in the traditional fluororubber compositions can give a low-friction fluororubber crosslinked product that is well balanced and improved in properties such as low frictional properties, low tackiness and low resilience properties and is favorably employed for fabrication of parts including leakproof rubber parts represented by O-rings, packings and gaskets, rubber vibration insulators, belts, rubber-coated fabrics, and impact-absorbing stoppers as stoppers in HDD. The present invention has been completed based on these findings.
Known technologies related to the polyol vulcanization of fluororubbers include the following.
JP-B-H04-37094 discloses a process of surface modification of a fluorine-containing elastomer formed product wherein a surface of a vulcanized fluorine-containing elastomer formed product is impregnated with a crosslinking (vulcanizing) agent polyhydroxy compound and optionally a vulcanization accelerating promoter (crosslinking accelerator) to perform re-vulcanization. This surface modification process is described to provide non-tackiness and low frictional properties in the surface of vulcanized fluorine-containing elastomer formed product.
However, the process employs a surface treatment solution of the crosslinking agent and crosslinking accelerator in an organic solvent such as acetone for impregnating the surface with the crosslinking agent polyhydroxy compound and vulcanization accelerating promoter (crosslinking accelerator). The use of organic solvent possibly leads to environmental pollution. Moreover, the surface treatment after vulcanization adds a process to increase the cost.
JP-A-H07-3099 discloses a fluororubber composition comprising (A) 100 parts by weight of a fluororubber obtained by copolymerizing vinylidene fluoride, hexafluoropropylene and optionally tetrafluoroethylene, (B) 0.05 to 2 parts by weight of an organic quaternary phosphonium salt, (C) 0.01 to 3 parts by weight of a nitrogen-containing organic compound and/or a phosphorous-containing organic compound, (D) 0.1 to 10 parts by weight of a polyhydroxy compound, and (E) 0.5 to 30 parts by weight of a metal oxide and/or a metal hydroxide. It is described that the composition may be preformed into a desired shape, be placed in a mold, and be compression molded under heating to perform polyol vulcanization, giving a vulcanized fluororubber formed product. The thus-formed product is described to maintain mechanical characteristics and be free of forming failure during the vulcanization forming.
However, the formed products disclosed in Examples of the patent document show insufficient migration of the organic quaternary phosphonium salt to the surface of rubber formed product, and consequently have high friction coefficient and tackiness.
JP-A-H07-3100 discloses a fluororubber composition comprising (A) 100 parts by weight of a fluororubber obtained by copolymerizing vinylidene fluoride, hexafluoropropylene and optionally tetrafluoroethylene, (B) 0.05 to 2 parts by weight of an organic quaternary phosphonium salt, (C) 0.01 to 3 parts by weight of an organic quaternary ammonium hydrogen sulfate, (D) 0.1 to 10 parts by weight of a polyhydroxy compound, and (E) 0.5 to 30 parts by weight of a metal oxide and/or a metal hydroxide. It is described that the composition may be preformed into a desired shape, be placed in a mold, and be compression molded under heating to perform polyol vulcanization, giving a vulcanized fluororubber formed product. The thus-formed product is described to have good mechanical properties and be free of forming failure during the vulcanization forming.
However, as in the case of the above patent document 2, the formed products disclosed in Examples of this patent document show insufficient migration of the organic quaternary phosphonium salt to the surface of rubber formed product, and consequently have high friction coefficient and tackiness.
JP-A-H07-82449 discloses a polyol-vulcanizable fluororubber composition comprising a polyol-vulcanizable fluororubber and a compound analogous to hydrotalcite. According to the patent document, vulcanization is performed in a manner such that the fluororubber is mixed with vulcanizing ingredients and the mixture is subjected to primary vulcanization by press vulcanization at 140 to 200° C. for about 2 to 120 minutes and to secondary vulcanization by oven vulcanization at about 150 to 250° C. for about 0 to 30 hours. The vulcanizing ingredients mentioned in the patent document include the following, which are used in the amounts described per 100 parts by weight of the fluororubber: a vulcanizing agent (e.g., a polyhydroxy aromatic compound) used in an amount of 0.5 to 10 parts by weight, an acid receiver (e.g., a bivalent metal oxide or hydroxide) used in an amount of 1 to 20 parts by weight, and a vulcanization accelerator (e.g., a quaternary ammonium or phosphonium salt) used in an amount not more than 10 parts by weight. The composition is described to have good mold releasability, excellent vulcanization properties and improved engine oil resistance.
However, the formed products disclosed in Examples of this patent document show insufficient migration of the vulcanization accelerator (such as a quaternary phosphonium salt) to the surface of rubber formed product, and consequently have high friction coefficient and tackiness. Moreover, the use of the vulcanization accelerator, such as an organic quaternary phosphonium salt, in an increased amount in combination with the hydrotalcite-analogous compound as described in the patent document results in a higher crosslinking rate such that the composition is crosslinked before being poured into a mold in the press molding and cannot be formed into a desired shape.
JP-A-2000-34379 discloses a fluororubber composition comprising a raw-material fluororubber, a polyol vulcanizing agent, an organic promoter, a vulcanization supplement accelerator, an acid receiver and optionally a filler, wherein the vulcanization supplement accelerator is calcium hydroxide that is treated with a fatty acid ester or the like and has an average particle diameter of not more than 7.5 μm and a specific surface area of not less than 20 m2/g. The composition is described to give a vulcanized product excellent in resistance to permanent compression set. According to the patent document, the vulcanization is performed in a manner such that primary vulcanization is carried out at 170° C. for 10 minutes and secondary vulcanization at 200° C. for 24 hours.
However, fluororubber formed products obtained according to this patent document have increased crosslinking density and impact resilience, presumably because of the fact that the vulcanization supplement accelerator has a high specific surface area and is treated with a fatty acid ester or the like. Moreover, the formed products show insufficient migration of the organic accelerator (vulcanization accelerator) such as an organic quaternary phosphonium salt to the surface of rubber formed product. Furthermore, Examples in this patent document result in high friction coefficient and tackiness.
JP-A-2001-192482 discloses a process in which a fluororubber composition is vulcanized and formed in the presence of a polyol vulcanizing agent and is heat treated at a temperature of about 250 to 300° C. to give a vulcanized fluororubber formed product having superior properties such as resistance to permanent compression set, wherein the fluororubber composition comprises 100 parts by weight of a fluororubber, 0.5 to 3 parts by weight of calcium hydroxide, 4 to 15 parts by weight of magnesium oxide, and 10 to 50 parts by weight of thermal black and a bituminous coal filler combined. The process employs the polyol vulcanizing agent, such as a polyhydroxy aromatic compound, in an amount of about 0.5 to 10 parts by weight per 100 parts by weight of the fluorine-containing rubber, and an ammonium or phosphonium salt in an amount of about 0.1 to 30 parts by weight. According to the patent document, the fluororubber composition is vulcanized and formed using a compression press or the like at about 150 to 230° C. for about 1 to 30 minutes; the products for use as grommets and seal packings are further heat treated (secondary vulcanization) at about 250 to 300° C. for about 5 to 48 hours in an air oven or the like.
However, the formed products disclosed in this patent document have high impact resilience, and show insufficient migration of the organic quaternary phosphonium salt to the rubber surface and consequently exhibit high friction coefficient and tackiness.
The fluororubber compositions and vulcanized formed products thereof proposed so far further include the following.
Japanese Patent No. 3063172 (corresponding to JP-A-H04-236254) discloses a fluororubber composition comprising 100 parts by weight of a fluororubber and 0.5 to 10 parts by weight of a liquid hydrocarbon rubber selected from liquid polyisoprene rubbers and hydrogenated liquid polyisoprene rubbers. The composition is described to be excellent in processability such as extrusion properties and in vulcanized rubber properties.
Japanese Patent No. 3222054 (corresponding to JP-A-H09-208751) discloses a rubber composition comprising (A) a fluororubber polymer, (B) a wax containing a fluorine-containing organic group and having a melting point of 30 to 200° C., and (C) a crosslinking agent selected fromamines, polyols and peroxides. The rubber composition is described to possess excellent workability, kneading processability and mold releasability, and to give a formed product comparable in properties to the existing products.
Japanese Patent No. 2653340 (corresponding to JP-A-H06-293850) discloses a fluororubber composition comprising (A) a polyol-crosslinkable fluororubber, (B) a liquid fluororubber, and (C) a polyol in which at least one OH group in the molecule is silylated. It also describes that the composition may be crosslinked at a low temperature of 130 to 160° C. (primary crosslinking) to avoid foaming, and be subjected to secondary crosslinking at a temperature of 120 to 250° C. The composition is described to be excellent in workability and to give a formed product having low hardness and widespread uses.
JP-A-H05-239300 discloses a vulcanizable fluoroelastomer composition comprising (A) an elastomer copolymer having a vinylidene fluoride unit and at least one fluorine-containing monomer unit, (B) a tertiary phosphine vulcanization accelerator, such as triphenylphosphine, substituted with an alkoxyl or phenoxy group, (C) a polyol crosslinking agent, and (D) a bivalent metal oxide or hydroxide. The composition is subjected to primary vulcanization (press vulcanization) and secondary vulcanization (oven heating) to provide a vulcanized formed product, such as a sealing material. The primary vulcanization rate is high, and the vulcanized formed product has good rubber elasticity and tensile properties.
JP-A-H06-248145 discloses a fluororubber composition comprising a fluororubber, calcium oxide and polyethylene wax, wherein the fluororubber is obtained by copolymerizing vinylidene fluoride, tetrafluoroethylene and propylene. The patent document teaches that the composition may be vulcanized at 100 to 400° C. for several seconds to 5 hours, and be subjected to secondary vulcanization at 150 to 300° C. for about 30 minutes to 48 hours to stabilize properties of the vulcanizate. According to the disclosure of the document, the composition is free of fusion failure in the vulcanization and forming, and the formed product has high heat resistance.
JP-A-H06-306180 discloses a process for producing a vulcanized fluororubber formed product, wherein a fluororubber obtained by copolymerizing vinylidene fluoride, hexafluoropropylene and optionally tetrafluoroethylene, is formed by polyol vulcanization in a mold with use of an organic quaternary ammonium salt as vulcanization accelerator. The document describes that the vulcanization forming can be performed without forming failure.
The fluororubber composition disclosed in these patent documents include a liquid hydrocarbon rubber, a wax containing a fluorine-containing organic group and having a melting point of 30 to 200° C., a liquid fluororubber, a tertiary phosphine crosslinking accelerator, calcium oxide, a polyethylene wax, or an organic quaternary ammonium salt. Vulcanized formed products from the fluororubber compositions are unsatisfactory in low tackiness, low frictional properties and low impact resilience, and in balance of such properties.